The electric vehicle and hybrid vehicle provided with a motor as a source of driving force have attracted attention in recent years. These vehicles are provided with a battery having relatively high voltage for supplying electric power to the motor, in which a relay is disposed between the battery and the motor for connection and disconnection of the power supply.
During power-up, the relay is activated to supply the electric power to the motor. If a large current flows when the relay is turned on, an electric discharge may occur between the movable contact and the fixed contact to cause the contacts to be welded. The welding of the relay contacts causes a problem that the power supply cannot be shut down. Thus, as indicated in Japanese Patent Laying-Open No. 2001-327001, for example, the hybrid vehicle and the like are subjected to abnormality diagnosis for the power supply system including the relay at the time of activation of the system.
In recent years, it has been contemplated that a large-capacity power storage condenser (hereinafter referred to as a capacitor) is used as a power storage device in place of the battery. It has also been contemplated that the battery and the capacitor having different characteristics from each other are used in combination in the vehicle.
The capacitor has high voltage across its terminals as in the battery and is capable of accumulating the energy that allows the motor to be operated. Therefore, it is desirable to provide a relay that allows the capacitor and the motor to be disconnected while the vehicle is stopped.
However, as in Japanese Patent Laying-Open No. 2001-327001, the relay on the battery side is checked and, after that, the relay on the capacitor side is similarly checked, which causes a problem of being time-consuming. In other words, after the relay on the battery side is checked, the node on the load side to which the smoothing condenser is connected is once discharged. This requires the node on the load side to be again charged before the relay on the capacitor side is checked.